The present invention relates to a certain kind of solar collector plant, and more precisely an operating method for a special solar collector plant. The method comprises use of a control unit for a liquid pump in the plant, and one aspect of the invention concerns such a control unit also.
The special solar collector plant referred to here, is disclosed in applicant's own prior Norwegian patent application no. 94.3257, which relates to a solar collector having a radiation absorber with a channel system for conducting a liquid which removes heat from a surface layer heated by incident sunbeam radiation. The special feature of this prior application is that the liquid channels in the absorber are filled with particles, e.g. globular, ceramic particles with dimension in the range 2-4 mm, which particles lift the liquid by means of a capillary effect to contact with the surface layer above. Thereby a good thermal contact with the surface layer is ensured without simultaneously filling the channels with the liquid, and hence a build-up of a liquid pressure in the channel system is avoided.
However, there exists a problem with the prior device, namely that when liquid is supplied from above through a distributor at the channel starting points on the top (usually the absorber has a slanted position), the liquid will normally not be distributed in a perfect manner to every one of the through-going and particle-filled channels in the absorber. The liquid will only flow through some of the channels, which leads to a situation where less than the complete surface layer is subject to effective cooling. This state seems to be metastable, and it may remain unchanged for a period of as much as 1-2 hours. The reason is that the space between the particles in the channels is originally filled with air. When a liquid is supplied, the air is partly expelled, and it is brought along with the liquid down through the channel and out of the absorber. At the same time this leads to a small increase of pressure in the air in the channels where liquid is not flowing, and this seems to be the mechanism which maintains the metastable state. Thus, the problem to be solved is to achieve disruption of the metastable state as soon as possible, so that liquid filling can be effected more rapidly in all channels.
From U.S. Pat. No. 4,414,961 there is known an operating method for a solar collector plant of a somewhat different type than the plant according to NO-94.3257, but with some similar features, e.g. the use of a liquid circulation pump working in a start-up phase and a continuous operation phase.
Further, U.S. Pat. No. 4,191,166 discloses a control unit for a pump providing liquid circulation in a solar collector plant, which control unit comprises a computer having a clock for controlling switch-on and shut-down times for the pump.
However, none of the above U.S. patents teach pumping systems that will be able to remove air efficiently from a solar collector plant of the general type disclosed in the above mentioned NO-94.3257, in which the liquid channels in the radiation absorber unit are filled with e.g. globular, ceramic particles.